UTILITY OF SIGNAL PROCESSING IN DETECTION OF HUMAN VISCEROSENSORY PERCEPTION

E.L. Fallen¹, M. Kamath¹, G. Tougas², A. Upton^3
1Division of Cardiology,  ²Division of Gastroenterology,
³Division of Neurology, Department of Medicine, McMaster University Faculty of Health Sciences, Hamilton, Ontario, Canada

Abstract: Using a specially designed esophageal catheter/stimulator probe we were able to gain access to vagal afferent fibers in the distal esophagus. Our studies on the effect of vagal afferent electrostimulation on both cerebral evoked potentials (EvP) and the power spectrum of heart rate variability in healthy control subjects yielded the following observations:

1. Vagoafferent stimulation increases the high frequency (HF) power and reduces the low frequency (LF) power.
2. This effect is constant across stimulation frequencies from 0.1 to 1.0 Hz and across stimulation intensities from 2.5 to 20 mA.
3. There is a linear correlation between stimulation intensity and the amplitude of EvP in contrast to a non-linear relationship with short term power spectral indices.
4. While EvPs are elicited only at stimulation intensities above perception threshold, there is already a significant shift of the power spectrum to increased vagal modulation.

These studies support the concept that power spectral indices of heart rate variability represent phasic output responses to tonic afferent viscerosensory signals in humans. The constancy of the vagoefferent response suggests non-linear negative feedback through central integrative processing.

INTRODUCTION

Activation of vagal afferent receptors elicit tonic impulses that are fundamentally inhibitory to central sympathetic nerve discharge (1). The frequency composition of the beat to beat heart rate series can be viewed as the phasic or modulatory output of central integrative responses to these tonic input signals. Because the lower esophageal mucosa is richly innervated with vagal afferent fibers, we modified a 12 French manometric esophageal catheter with a custom-designed stimulating electrode (2).  With the application of 21 scalp electrodes to measure cerebral evoked potentials and continuous ECG recordings for computation of the heart rate autospectra, we set out to explore, non-invasively, the effect of vagoafferent stimulation (viscerosensory signal input) on both central processing and peripheral autonomic efferent responses.

METHODS

In 13 healthy male volunteers (age 24±6 years) we accessed vagal afferent fibers in the distal esophagus using a specially modified 12 French manometric catheter fitted with a custom-designed stimulating electrode at the tip (16 mA, 200 ms square wave stimulus at 0.2 Hz for 128 s) We varied, in random order, electrical stimulation frequencies from 0.1 to 1 Hz. In another group of 10 healthy male subjects we applied 21 scalp electrodes according to the 10/20 international system to measure the amplitude and latencies of EvP (3). In this group we varied the stimulation intensities from 2.5 to 20 mA.

RESULTS

There was a dramatic shift in the power spectrum to a dominant vagal modulatory effect in response to both esophageal electrostimulation frequencies and intensities. Of interest the enhanced cardiac vagal modulation was seen equally at all frequencies and intensities beginning with intensities well below perception threshold. Conversely, there was a linear correlation between stimulation intensities and EvP amplitudes but no change until the subjects first perceived the sensation at 8-10 mA.  There was a small but significant decrease in heart rate (62±6 to 58±5 bpm, avg -4bpm) at maximal intensities.

DISCUSSION

The observation that cerebral evoked potentials could only be elicited above perception thresholds implies that viscerosensory afferent impulses project not only to brain stem nuclei but appear to involve higher centers including thalamic and cortical neural pathways. On the other hand, cardiac efferent responses are largely independent of cortical neural projections since the shift in sympathovagal balance occurred at intensities well below perception threshold. Moreover the pattern of output to the heart corresponds to a non-linear process, presumably as a result of post central processing at sub-cortical centers. That is, the constancy of each power spectral component across a wide range of stimulation intensities favors  a central integrative process capable of converting tonic afferent impulses into phasic or modulatory efferent output.

REFERENCES

 [1] V.S. Bishop, E.M. Hasser et al. “Arterial and cardiopulmoinary reflexes in the regulation of the neurohumoral drive to the circulation”.  Fed Proc. 44:2377, 1985.

[2]    G. Tougas, M.V. Kamath et al. “Modulation of neurocardiac function by esophageal stimulation in humans”. Clin Sci.  92:167, 1997.

[3]   G. Tougas, P. Hudoba et al. “Cerebral evoked potential responses following direct vagal and esophageal electrical stimulation in humans”. Am J Physiol 264:G486, 1993.